In a thermal printer, a dye carrier member containing one or more dye colors is disposed between a receiver member, such as paper, and a print head assembly formed of one or more thermal elements often referred to as thermal pixels. When a thermal pixel is energized, the heat produced causes a dye color from the dye carrier member to transfer to the receiver member engaging an outside surface of a rotatable drum. The density (darkness) of the printed dye is a function of the temperature of the thermal pixel and the time the dye carrier member is heated (the energy delivered from the thermal pixel to the dye carrier member). Thermal dye transfer printers offer the advantage of true "continuous tone" dye density transfer. This transfer is obtained by varying the energy applied to each thermal pixel which results in a variable dye density image pixel on the receiver member.
A first type of print head is formed with a plurality of resistive thermal elements forming the thermal pixels. The thermal pixels are usually organized into a plurality of groups of thermal pixels. The thermal pixels in each group are simultaneously addressed in parallel, and each group is addressed sequentially one at a time. In this manner, a smaller and less expensive power supply is needed than required when all of the thermal pixels are energized at the same time. In this regard see, for example, U.S. Pat. No. 4,621,271 (S. A. Brownstein, issued on Nov. 4, 1986) which describes method and apparatus for controlling a thermal printer arranged with a plurality of groups of thermal pixels. When a group of thermal pixels are addressed, the thermal pixels are each selectively energized and are driven by a constant voltage. More particularly, a technique is described which addresses the thermal pixels of each group a plurality of N times during a line printing period, and has means for selectively energizing each of the thermal pixels of each group when they are addressed. In this manner each thermal pixel supplies thermal energy to the dye carrier member which substantially corresponds to a desired dye color density to be reproduced in an image pixel on the receiver member.
A second type of thermal printer employs one or more laser beams which are each selectively energized as the beam impinges or scans the surface of the dye carrier member past each thermal pixel area. The heat which is provided as the laser beam impinges the dye carrier member in each pixel area determines the density level (amount of dye color transferred) on the receiver member in the pixel area. An exemplary thermal dye transfer printing apparatus using an array of semiconductor diode lasers is disclosed, for example, in U.S. Pat. No. 4,804,975 (K. Yip, issued on Feb. 14, 1989). Means are provided for controlling the laser diodes to produce light and modulate the light from the individual lasers to provide sufficient energy to cause different amounts of dye to transfer from the dye carrier member to the receiver member and form pixels with different levels of density.
With laser thermal printers, precise pixel resolution on the receiver member is required under certain conditions as, for example, for creating 4-color proofs (defined in the graphic arts industry as the output from the thermal printer). In order to print dots (micropixels) at, for example, 1800 and 2400 dots per inch (dpi), it is desirable that the laser thermal printer maintain a small fractional part of micropixel resolution. When writing onto, for example, a receiver member wrapped about a writing drum, synchronization of pixel timing must repeat at regular intervals. Slight changes in rotational speed or pixel timing has a cumulative effect in compromising the accuracy needed for creating the 4-color image. This requirement for accurate synchronization of the pixel timing is compounded by the need to be able to print dots at more than one dpi value (e.g., 1800 and 2400 dpi). The problem is to provide a thermal printer capable of providing such micropixel resolution.